Referring to FIG. 1, a conventional light guide film 1 is adapted for use with a light source 2 for guiding light emitted therefrom. The conventional light guide film 1 has a substrate 11 having opposite top and bottom surfaces 111, 112. When the light emitted from the light source 2 enters the conventional light guide film 1, it will propagate within the conventional light guide film 1 by total internal reflection. The bottom surface 112 of the conventional light guide film 1 is usually formed with a plurality of microstructures (not shown) that are periodically arranged for altering the incident angle of the light at the top surface 111, so as to allow the light to exit the conventional light guide film 1 from the top surface 111. However, since the conventional light guide film 1 is relatively thin comparing to the size of the light source 2, part of the light emitted from the light source 2 will not enter the conventional light guide film 1 and result in energy loss.